Resin lubrication grease composition

ABSTRACT

The present invention provides a grease composition to be used for lubrication between resin-made members or lubrication between a resin-made member and a member made from another material. With respect to the total mass of the composition, more than 0.5 mass % but less than 7.5 mass% of a solid organic molybdenum compound is contained.

TECHNICAL FIELD

The present invention relates to a resin lubrication grease composition. More specifically, the present invention relates to a resin lubrication grease composition suitable to use for lubrication between resin-made members or between a resin-made member and a member made from another material, for example, a metal-made member.

BACKGROUND ART

Various metal-made members have conventionally been used as automobile components and the like, but these days, resin-made members are increasingly being used instead of metal-made members for the purpose of reducing the weights. For this reason, both resin-made members and metal-made members are currently used. For example, a resin (polyamide)-made worm wheel gear and a steel-made worm gear are used in a reducer part of electric power steering of automobiles.

As a grease composition to be used for lubrication between resin-made members and between a resin-made member and a metal-made member, Patent Literature 1 describes a lubrication grease composition containing a polyolefin wax. When used for resin lubrication parts, this grease composition can achieve low friction and can meet the demand for improvements in efficiency of components having resin lubrication parts. Patent Literature 2 describes a resin lubrication grease composition characterized in that the grease containing a thickener and a base oil is made contain a montan wax. This grease composition is excellent in that the grease composition can reduce the coefficient of static friction of a lubrication part to extend the durability life of the lubrication part.

However, in the case where these grease compositions are applied to a reducer part of electric power steering, when the steering wheel is slightly turned while the steering is not assisted by a motor, like during the driving on a highway, for example, the lubrication part of the reducer slides, causing a problem that the steering wheel is displaced from the center in some cases, due to the low coefficient of static friction.

CITATION LIST Patent Literatures

-   Patent Literature 1: Japanese Patent Application Publication No. Hei     9-194867 -   Patent Literature 2: Japanese Patent Application Publication No.     2002-371290

SUMMARY OF INVENTION Problems to Be Solved by the Invention

Therefore, an object of the present invention is to provide a resin lubrication grease composition that can increase the coefficient of static friction of a lubrication part, and suppress an increase in the coefficient of dynamic friction associated with the increased coefficient of static friction and maintain the coefficient of dynamic friction at a certain low level, thus suppressing sliding of the lubrication part of a reducer when no assistance is provided by a motor and maintaining a lubrication efficiency when assistance is provided by the motor.

Means for Solution of the Problems

The present invention is a resin lubrication grease composition characterized in that the resin lubrication grease composition comprises a solid organic molybdenum compound. Specifically, the present invention provides a grease composition as described below.

-   [1] A grease composition to be used for lubrication between a     resin-made member and a resin-made member or lubrication between a     resin-made member and a member made from another material,     comprising:     -   a solid organic molybdenum compound in an amount of more than         0.5% by mass and less than 7.5% by mass relative to a total mass         of the composition. -   [2] The grease composition according to the above [1], wherein     -   the content of the solid organic molybdenum compound is 1 to 5%         by mass. -   [3] The grease composition according to the above [1] or [2],     wherein     -   the thickener is a lithium soap thickener. -   [4] The grease composition according to any one of the above [1] to     [3], wherein     -   a base oil of the grease composition contains at least one         compound selected from the group consisting of synthesized         hydrocarbon oils and mineral oils. -   [5] The grease composition according to any one of the above [1] to     [4], wherein     -   the resin-made member to be lubricated is of a polyamide. -   [6] The grease composition according to any one of the above [1] to     [5], wherein     -   the member made from another material other than the resin-made         member to be lubricated is a metal-made member. -   [7] A reducer for an electric power steering, filled with the grease     composition according to any one of the above [1] to [6].

Advantageous Effects of Invention

The present invention makes it possible to provide a resin lubrication grease composition that can increase the coefficient of static friction of a lubrication part and maintain the coefficient of dynamic friction at a certain low level, thus suppressing sliding of the lubrication part of the reducer when no assistance is provided by a motor and maintaining the lubrication efficiency when assistance is provided by the motor.

DESCRIPTION OF EMBODIMENTS Thickener

The thickener to be used in the grease composition of the present invention is not particularly limited. For example, the thickener includes soap thickeners represented by a Li soap and a Li complex soap, urea thickeners represented by diurea, inorganic thickeners represented by organoclay and silica, and organic thickeners represented by PTFE. Among these, soap thickeners are preferable. Since soap thickeners themselves have low coefficients of dynamic friction, an excellent lubrication efficiency can be achieved by using a soap thickener. Lithium 12-hydroxystearate, lithium stearate, and a Li complex soap (for example, a Li complex soap formed from 12-hydroxystearic acid and azelaic acid) are more preferable, and lithium 12-hydroxystearate and a Li complex soap are particularly preferable. Since thickening can be made with a small amount of the thickener by using lithium 12-hydroxystearate and a Li complex soap, an excellent low-temperature operability can be achieved.

The amount of the thickener in the grease composition of the present invention is preferably 3 to 20% by mass, further preferably 4 to 17% by mass, and particularly preferably 5 to 15% by mass based on the total mass of the composition. By setting the amount to 3% by mass or more, it is possible to achieve a thickener effect sufficient for forming a grease, and to suppress leakage of the grease from lubrication parts. By setting the amount to 20% by mass or less, it is possible to provide the grease composition with a hardness appropriate to flow into lubrication parts.

Base Oil

The base oil to be used in the grease composition of the present invention is not particularly limited. For example, it is possible to use mineral oils, ester synthetic oils represented by diester and polyolester, synthesized hydrocarbon oils represented by poly-α-olefin (“PAO”), co-oligomer of ethylene and α-olefin, and polybutene, ether synthetic oils represented by alkyl diphenyl ethers and polypropylene glycol, silicone oil, fluorinated oil, and the like. Two or more of these may be used in combination.

Meanwhile, since the electric power steering has a smaller output than hydraulic power steering, the operation torque of the electric power steering at a low temperature is large, and operation failure sometimes occurs. In the case of using the grease composition of the present invention for a lubrication part of electric power steering, for example, a reducer part, when a synthetic oil is contained as the base oil, it is possible to reduce the operation torque at a low temperature. Particularly, a synthesized hydrocarbon oil is preferably contained, and poly α-olefin is more preferably contained. The proportion of the synthetic oil in the base oil is not particularly limited, but the proportion of PAO may be set to preferably 50 to 100% by mass, more preferably 60 to 100% by mass, and further preferably 70 to 100% by mass in order to achieve a favorable low-temperature operability. In this case, the type of the balance of the base oil is not a matter, but a mineral oil is preferable from the viewpoint of costs. Hence, in the case of using the grease composition of the present invention for a lubrication part of electric power steering, for example, a reducer part, the proportion of the mineral oil in the base oil is preferably 0 to 50% by mass, more preferably 0 to 40% by mass (for example, 20 to 40% by mass), and further preferably 0 to 30% by mass. As the mineral oil, any mineral oil can be used. The mineral oil includes, for example, refined mineral oils, high viscosity index oils, and dewaxed mineral oils. Dewaxed mineral oils are particularly preferable from the viewpoint of low-temperature operability.

The kinematic viscosity of the base oil at 40° C. is preferably 20 to 80 mm²/s, and more preferably 30 to 70 mm²/s, from the viewpoint of low-temperature operability and durability.

It is particularly preferable that the base oil contain 70 to 100% by mass of PAO having a kinematic viscosity at 40° C. of 15 to 420 mm²/s based on the total amount of the base oil. This makes it possible to obtain a grease composition particularly excellent in low-temperature operability. In this case, the kinematic viscosity at 40° C. of the entire base oil is preferably 30 to 70 mm²/s.

The content of the base oil in the grease composition of the present invention is an amount that is normally used for production of greases, and is, for example, 75 to 95% by mass, and is preferably 83 to 96% by mass, and more preferably 85 to 95% by mass, from the viewpoint of low-temperature operability.

Additive

Organic molybdenum compounds used as additives for greases in general include those which are solid and liquid at ordinary temperature and ordinary pressure. Since a solid organic molybdenum compound keeps the solid state even when blended in grease, the solid organic molybdenum compound can exhibit a different performance from a liquid organic molybdenum compound.

The solid organic molybdenum compound to be used in the present invention is not particularly limited as long as the solid organic molybdenum compound is solid at ambient temperature (for example, 25° C.).

The amount of the solid organic molybdenum compound in the composition of the present invention is more than 0.5% by mass and less than 7.5% by mass, and is preferably 1 to 5% by mass, and further preferably 2 to 3% by mass. When the amount is more than 0.5% by mass, the coefficient of static friction can be significantly increased, making it possible to suppress sliding of lubricated members. When the amount is less than 7.5% by mass, favorable low-temperature operability and lubrication efficiency of lubricated members can be maintained.

Various additives may be added to the grease composition of the present invention as necessary. Such additives include, for example, antioxidants, rust inhibitors, metal corrosion inhibitors, oiliness improvers, anti-wear agents, extreme pressure agents, solid lubricants. and the like. Specifically, the antioxidants include amine-based, phenol-based, quinoline-based, sulfur-based antioxidants, zinc dithiophosphates, and the like, and amine-based antioxidants are preferable. The amine-based antioxidants include, for example, phenyl α-naphthylamine, alkylphenyl a-naphthylamine, alkyl diphenylamine, and the like, and alkyl diphenylamine is particularly more preferable.

The rust inhibitors include zinc-based, carboxylic acid-based, carboxylate-based, amine-based, and sulfonate-based rust inhibitors, but sulfonic acid salt-based rust inhibitors, particularly Ca sulfonates, are preferable. The Ca sulfonates include calcium salts of petroleum sulfonic acid obtained by sulfonation of aromatic hydrocarbon components in lubricant oil distillates, calcium salts of synthetic sulfonic acids such as dinonylnaphthalenesulfonic acid and alkyl benzenesulfonic acids, overbased calcium salts of petroleum sulfonic acids, overbased calcium salts of alkyl aromatic sulfonic acids, and the like, and overbased Ca sulfonate is particularly preferable.

The metal corrosion inhibitors include thiadiazole-based, benzimidazole-based, and benzotriazole-based metal corrosion inhibitors, but benzotriazole-based metal corrosion inhibitors are preferable. The benzotriazole-based inhibitors include 1,2,3-benzotriazole, 1,H-benzotriazole, 4-methyl-1,H-benzotriazole, 4-carboxy-1,H-benzotriazole, sodium tolyltriazole, 5-methyl-1,H-benzotriazole, benzotriazole butyl ether, silver benzotriazole, 5-chloro-1,H-benzotriazole, 1-chloro-benzotriazole, 1-di(C8H17)aminomethyl-benzotriazole, 2,3-dihydroxypropyl-benzotriazole, 1,2-dicarboxyethyl-benzotriazole, (C8H17)aminomethyl-benzotriazole, bis(benzotriazole-1-yl-methyl) (C8H17)amine, N,N-bis(2-ethylhexyl)-4-methyl-1H-benzotriazole-1-methyl amine, N,N-bis(2-ethylhexyl)-5-methyl-1H-benzotriazole-1-methylamine, and the like, and 1-[N,N-bis(2-ethylhexyl)aminomethyl]-4-methylbenzotriazole is particularly preferable.

The oiliness improvers include fatty acids, fatty acid esters, and phosphoric acid esters.

The anti-wear agents and the extreme pressure agents include phosphorus-based, sulfur-based, and organic metal-based agents.

The solid lubricants include oxidized metallic salts, molybdenum disulfide, polytetrafluoroethylene, melamine cyanurate, and graphite.

The content of these optional additives is, for example, 0.1 to 10% by mass, preferably 0.1 to 5% by mass, and more preferably 0.5 to 3% by mass, based on the total mass of the composition of the present invention.

The 60-stroke worked penetration of the grease composition of the present invention is preferably 190 to 415, and more preferably 235 to 370, from the viewpoint of oil release, leakage prevention, and low-temperature operability.

The grease composition of the present invention can be easily produced by mixing the above components and other additives in a desired blending ratio in accordance with a conventional method.

The grease composition of the present invention is used for lubrication between resin-made members or between a resin-made member and a member made from another material, for example, a metal-made member. Specifically, the grease composition of the present invention can be favorably used for rolling-element bearings, ball screws, reducers of electric power steering devices, support yokes, and/or the like. The resin contained in resin-made members is not particularly limited, but a polyamide is preferable from the viewpoint of strength and hardness.

EXAMPLES Test Grease Test Grease Composition Containing Lithium 12-Hydroxystearate as a Thickener

First, 570.0 g of 12-hydroxystearic acid was completely dissolved in 2352.3 g of a base oil at 90° C. In a different container, 77.7 g of lithium hydroxide was completely dissolved in 388.5 g of pure water at 90° C., and both were mixed while the temperature was increased to 220° C., and thereafter the mixture was cooled to 100° C. or less while agitating, and the resultant was used as a base grease (the total amount of the base grease was 3000 g, the amount of the thickener was 19%).

A solid organic molybdenum compound was blended with the above base grease in the proportion shown in Table 1, and an additional base oil was added such that the proportion of the thickener became the amount shown in Table 1, followed by dispersion using a three roll mill to prepare a test grease composition. The consistency of the test grease composition was 325.

Test Grease Composition Containing Lithium Stearate as a Thickener

First, 957.2 g of stearic acid and 151.3 g of lithium hydroxide monohydrate were added to 1891.5 g of the base oil while agitating, and thereafter the mixture was heated to 230° C. Thereafter, the mixture was cooled to 100° C. or less while agitating to obtain a base grease. A solid organic molybdenum compound was blended with the above base grease in the proportion shown in Table 1, and an additional base oil was added such that the proportion of the thickener became the amount shown in Table 1, followed by dispersion using the three roll mill to prepare a test grease composition. The consistency of the test grease composition was 325.

Test Grease Composition Containing Lithium Complex Soap as a Thickener

First, 445.9 g of 12-hydroxystearic acid was added to 1776.0 g of the base oil, and the solution was heated to a temperature (80 to 90° C.) at which the solution was turned into a completely transparent liquid state. To this, what was obtained by adding, heating and dissolving 64.8 g of lithium hydroxide monohydrate into 363.5 g of water in advance was added. Then, saponification reaction of 12-hydroxystearic acid was conducted while vigorously agitating to form a lithium salt of 12-hydroxystearic acid. Next, 507.7 g of the base oil and 139.4 g of azelaic acid were added, followed by continuously agitating to obtain a uniform state. To this, what was obtained by adding, heating, and dissolving 64.8 g of lithium hydroxide monohydrate into 363.5 g of water in advance was added, and saponification reaction of azelaic acid was conducted while vigorously agitating. Next, the processing was advanced to a heating step, where the content was gradually heated to 200° C. Thereafter, the content was cooled to 100° C. or less while agitating to obtain a base grease. A solid organic molybdenum compound was blended with the above base grease in the proportion shown in Table 1, and an additional base oil was added such that the proportion of the thickener became the amount shown in Table 1, followed by dispersion using the three roll mill to prepare a test grease composition. The consistency of the test grease composition was 325.

The components used to prepare the test grease compositions are as follows:

Base Oil

-   PAO: Poly-α-olefin, the kinematic viscosity at 40° C.: 56 mm²/s -   Mineral Oil: Naphthene-based mineral oil, the kinematic viscosity at     40° C.: 115 mm²/s

Note that the kinematic viscosity were measured in accordance with JIS K 2220 23.

Additives

-   Solid organic Mo: MoDTC (Trade name of Molyvan A, produced by     R.T.Vanderbilt Company Inc.) -   Polyolefin wax: Polyethylene wax (Trade name: LICOWAX PE190 P,     produced by Clariant Chemicals) -   Montan wax: Partially saponified montanic acid ester wax (Trade     name: LICOWAX OP FL, produced by Clariant Chemicals)

Note that mass% shown in Table 1 is a value based on the total mass of the grease composition, and the balance is the base oil.

The worked penetration of the test grease composition means the 60-stroke worked penetration and was measured in accordance with JIS K 2220 7.

Test Method

On the grease compositions of Examples and Comparative Examples, a test as described below was conducted, and the arithmetic means of coefficient of friction after stabilization was measured to evaluate the properties of these. The results are shown in Table 1.

Coefficient of Static Friction and Coefficient of Dynamic Friction (Bowden test)

Test Conditions

-   Test piece: a resin pin (diameter 5.0 mm, length 15 mm, nylon GF30%)     -   Plate (2.0 × 15 × 160 mm, surface roughness Ra=0.2 µm, S45C) -   Sliding velocity: 1 mm/s -   Stroke: 10 mm -   Load: 19.6 N -   Temperature: 25° C. -   Amount of grease applied: 0.2 \-mm thickness -   Determination criteria: coefficient of static friction     -   0.155 or more=o (Passed), less than 0.155=x (Failed) -   Determination criteria: coefficient of dynamic friction     -   0.150 or less=o (Passed), more than 0.150=x (Failed)

TABLE 1 Comp. Ex. Ex. Comp. Ex. Ex. Comp. Ex. Ex. Comp. Ex. Ex. Comp. Ex. 1 2 1 2 3 3 4 4 5 6 5 7 6 7 8 8 9 10 11 12 Thickener Type Lithium 12-hydroxystearate Li stearate Li complex soap Lithium 12-hydroxystearate Amount of Thickener mass% 6.5 15 7 6.5 Base oil mixing ratio % PAO 70 70 70 70 70 70 70 100 50 40 70 70 70 70 70 70 Mineral oil 30 30 30 30 30 30 30 0 50 60 30 30 30 30 30 30 Additives mass% Solid organic Mo 0 0.5 1 2 5 7.5 10 2 2 2 0.5 2 20 0.5 2 20 0 0 0 0 Liquid organic Mo 0 0 3 0 0 Polyolefin wax 0 0 0 3 0 Montan wax 0 0 0 0 3 Performa nce Static µ × × ○ ○ ○ ○ ○ ○ ○ ○ × ○ ○ × ○ ○ × × × × Dynamic µ ○ ○ ○ ○ ○ × × ○ ○ ○ ○ ○ × ○ ○ × × ○ ○ ○ 

1. A grease composition for use in lubrication between a resin-made member and a resin-made member or lubrication between a resin-made member and a member made from another material, comprising: a solid organic molybdenum compound in an amount of more than 0.5% by mass and less than 7.5% by mass relative to a total mass of the composition.
 2. The grease composition according to claim 1, wherein the content of the solid organic molybdenum compound is 1 to 5% by mass.
 3. The grease composition according to claim 1, wherein a thickener is a lithium soap thickener.
 4. The grease composition according to claim 1, wherein a base oil of the grease composition contains at least one compound selected from the group consisting of synthesized hydrocarbon oils and mineral oils.
 5. The grease composition according to claim 1, wherein the resin-made member to be lubricated is of a polyamide.
 6. The grease composition claim 1, wherein the member made from another material other than the resin-made member to be lubricated is a metal-made member.
 7. A reducer for an electric power steering, filled with the grease composition according to claim
 1. 